1. Field of the Invention
The present invention relates to a method of manufacturing compound aspheric lenses.
2. Description of the Related Art
Lenses used in optical apparatuses including cameras and microscopes are mostly made of glass lens. Glass lenses are manufactured, each to have a desired curvature, by machining glass blocks (also known as lens blanks) produced by press-forming molten glass. Methods of manufacturing resin lenses are also in practice, by which resin instead of glass is press-formed, injection-molded, teemed or fabricated otherwise. These methods have the advantage of a lower manufacturing cost because, once molds are produced, they can be used for a large number of lenses. However, resin lenses have the fatal disadvantage of the high susceptibility of their optical performance to temperature variations, and therefore they are not used for precision lenses.
Incidentally, there are lenses known as aspheric lenses, namely lenses whose surface is not spheric. Because of their excellent performance which spheric lenses cannot provide, aspheric lenses are found very useful. Main methods currently available for the manufacture of aspheric lenses from glass include grinding of blanks and direct pressing. However, grinding does not readily lend itself to mass production, while direct pressing has constraints in terms of the type of usable niter and the product size and requires expensive equipment.
To obviate these disadvantages, what are known as compound aspheric lenses have been developed. A compound aspheric lens consists of a thin (e.g., 5 to 100 μm) molded resin layer having an aspheric face and a glass lens which constitutes the main body. The glass lens constituting the main body has a spheric or rough aspheric face. Both components are available at low manufacturing costs. Such resin-bonded aspheric lenses, namely compound aspheric lenses, are manufactured by the following process, for instance. Thus a metallic mold having a desired aspheric face is placed horizontally, a prescribed quantity of ultraviolet-curable resin fluid is dropped onto the central part of this metallic mold, a glass lens having a spheric or aspheric face is placed over the metallic mold, the glass lens and the metallic mold are brought close to each other leaving a certain distance between them, the resin fluid is cured by irradiation with ultraviolet rays, and the cured resin is bonded to the glass lens to provide a compound aspheric lens.
The compound aspheric lens manufacturing method described above is sometimes susceptible to the entrance of bubbles (air bubbles) into the molded resin layer. The moment the resin fluid comes into contact with the metallic mold, the bubbles get in. Since bubbled products are rejected, this conventional process involves the problem of a low efficiency percentage. A process improved in efficient percentage was developed, and published in Japanese Patent No. 3191447.
The manufacturing method described in Japanese Patent No. 3191447 (a prior patented invention) consists of:
a first step of placing a glass lens having a spheric or rough aspheric face horizontally;
a second step of dropping a prescribed quantity of radiation-curable (ultraviolet-curable) resin fluid onto the central part of the glass lens;
a third step of turning upside down the glass lens;
a fourth step of mounting the glass lens over a “metallic mold having a desired aspheric face”;
a fifth step of bringing the glass lens and the metallic mold to positions at a prescribed distance to each other;
a sixth step of curing the resin fluid sandwiched between the glass lens and the metallic mold by irradiating it with rays (ultraviolet rays), and
a seventh step of separating the cured molded resin layer from its interface with the metallic mold.